A day in advance! (8)

I believe the answer is:

tomorrow

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'a day' is the definition.
(I know that tomorrow is a type of Earth day)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tomorrow that I've seen before include "when weekend begins?" , "... and ...and ... / Creeps in this petty pace from day to day (Macbeth)" , "They say it never comes" , "In the near future" , "___ Never Knows, 1966 Beatles song" .)

I've seen this clue in the Evening Standard.

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